1. Field of the Invention
This invention relates generally to tensiometers and more specifically to deep direct measurement of water pressure, moisture content, and water flux.
2. Description of Related Art
Water migration in shallow soils and rocks have been measured using tensiometers with water-filled tubes. This type of tensiometer is not useful at depths greater than about 7 meters below the soil surface because the water level in the measurement tube varies in a manner that cannot be known by the person at the surface taking the measurement as described by Daniel B. Stephens on pages 204 to 207 of his text, Vadose Zone Hydrology, (Lewis Publishers: Boca Raton, Fla. 1996).
In 1977 Dzekunov and Faybishenko (Device for determination of water pressure in soils (title translated from Russian), Certificate of Invention No. 591761, Moscow, 1977) designed a tensiometer having a porous ceramic wall at the bottom and an inverted solid cup some distance up from the ceramic bottom. This work is also described in a text by Faybishenko, entitled Water-Salt Regime of Soils under Irrigation (title translated from Russian), Moscow, Agropromisdat, 1986. The tensiometer was filled with water in a manner that left air trapped under the inverted solid cup. The lower porous wall allowed the water inside the tensiometer to come to equilibrium with the water in the surrounding soil. A second air space was left at the upper end of the tensiometer and the vapor pressure was measured to verify that the upper cell had not drained all its water. In the lower cell, when water leached out of the ceramic wall due to soil drying, the lower cell air pressure changed proportionately. Only two tubes extended from the tensiometer to the soil surface. One extended from the air phase in the upper cell and one from the air phase in the lower cell. The upper cell tube was used to replenish water and allow air to escape so that the pressure would not increase and force water into the tube that connected to the air volume in the lower cell. If that happened, the pressure measurement in the lower chamber was affected by the amount of the weight of water that entered the tube, and errors arose. The lower cell tube was used to measure lower cell air pressure. There were two major difficulties with this design. The first is that measurement of lower cell air pressure had to stop during the time upper-cell water was being replenished. In addition, the water had to be refilled very slowly and with very little control, so water was frequently forced into the lower cell air-pressure measurement tube. Secondly, in this design, the volume of air in the tube that extended from the lower-cell air pocket to the surface was too large for an effective instrument. The volume of air in the tube connecting the pressure sensor to the lower cell almost equaled the air in the confined volume of the lower cell. The result was a slow response time to pressure changes in the surrounding water, loss in sensitivity of measurement, and water column variations in the lower cell water column that sometimes became so great as to wash into the air pressure measurement tube.
Villa Nova et al. (Soil Technology, 2:403-407, 1989) designed a tensiometer having an air-pocket at the top of a water-filled tube connected to a porous tip. He used only one tube wherein the water level was required to be above the surface of the ground. His system is limited to measurement depths of 2 to 5 meters and measurements cannot be made remotely.
T. Tokunaga (Soil Science, 54(3):171-183, 1992) attempted measure the water level above a ceramic cup containing an air space. The water level is evaluated using measurements of the air pocket volume. Tokunaga performs calculations using Boyles Law to adjust for the water level below the ground surface. His apparatus is limited to manual measurements because the same tube is used for both water and air feed.
Problems with tensiometers designed for measuring water in the vadose zone include, uncontrolled water-level changes when water from the soil enters the tensiometer, and loss of isolation of the lower cell air volume during drying cycles. Thus, for currently available tensiometer systems to function properly, the maximum installation depth is limited to 5-7 meters. None of the currently available tensiometer systems are conducive to remote operation.